Tuesday, July 18, 2023

SPACE

Astronomers explore the chromosphere of peculiar white dwarfs

Observations explore the chromosphere of peculiar white dwarfs
Approximately 1 h of ULTRACAM g-band light curves for SDSS J1252, each taken on a
 different night. Credit: Farihi et al, 2023

Using the 3.6-m New Technology Telescope (NTT) at the La Silla Observatory in Chile, astronomers have observed three peculiar white dwarfs of the DAHe subtype. In their results, they found dipolar chromospheres in two of these objects. The findings were reported in a paper published July 5 on the preprint server arXiv.

White dwarfs (WDs) are stellar cores left behind after a star has exhausted its nuclear fuel. Due to their high gravity, they are known to have atmospheres of either pure hydrogen or pure helium. However, a small fraction of WDs shows traces of heavier elements.

DAHe (D: degenerate, A: Balmer lines strongest, H: magnetic line splitting, e: emission) is a relatively new and small class of magnetic white dwarfs that showcase Zeeman-split Balmer emission lines. To date, only a few dozen DAHe WDs are known. The first of them was GD 356—an isolated white dwarf discovered nearly 40 years ago.

A team of astronomers led by Jay Farihi of the University College London, U.K., decided to investigate three objects of this rare class, in order to better understand the nature of the entire population. For this purpose, they employed ULTRACAM—a frame-transfer CCD imaging camera mounted on the NTT telescope. The study was complemented by data from NASA's Transiting Exoplanet Survey Satellite (TESS).

"This study focuses on light curves and the resulting periodicities of three DAHe white dwarfs, using both ground- and space-based photometric monitoring," the researchers wrote.

The three observed DAHe WDs were: SDSS J125230.93−023417.7 (or SDSS J1252 for short), LP 705-64 and WD J143019.29−562358.3 (WD J1430). It turned out that the folded ULTRACAM light curves of SDSS J1252 and LP 705-64 exhibit alternating minima that are indicative of two distinct star spots 180 degrees out-of-phase during rotation. For WD J1430, the light curves reveal a single maximum and minimum.

The astronomers found that the amplitudes of the multi-band photometric variability reported for all the three DAHe  are all several times larger than that in GD 356. They noted that all the known DAHe stars have light curve amplitudes that increase toward the blue in correlated ratios, which points to cool spots that produce higher contrasts at .

According to the authors of the paper, their findings suggest that some magnetic WDs create intrinsic chromospheres as they cool, and that no external source is responsible for the observed temperature inversion.

"Given the lack of additional periodic signals and the compelling evidence of DAHe white dwarf clustering in the HR diagram (Walters et al, 2021; Reding et al, 2023; Manser et al, 2023), an intrinsic mechanism is the most likely source for the spotted regions and chromospheric activity," the researchers concluded.

More information: J. Farihi et al, Discovery of Dipolar Chromospheres in Two White Dwarfs, arXiv (2023). DOI: 10.48550/arxiv.2307.02543


Journal information: arXiv 


© 2023 Science X Network


Astronomers discover eight new cataclysmic variables




 Two White Dwarfs



SLEEPY



XRISM mission to study ‘rainbow’ of X-rays


Business Announcement

NASA/GODDARD SPACE FLIGHT CENTER

XRISM Spacecraft 

IMAGE: XRISM, SHOWN IN THIS ARTIST’S CONCEPT, IS AN X-RAY MISSION THAT WILL STUDY SOME OF THE MOST ENERGETIC OBJECTS IN THE UNIVERSE. view more 

CREDIT: NASA'S GODDARD SPACE FLIGHT CENTER CONCEPTUAL IMAGE LAB



A new satellite called XRISM (X-ray Imaging and Spectroscopy Mission, pronounced “crism”) aims to pry apart high-energy light into the equivalent of an X-ray rainbow. The mission, led by JAXA (Japan Aerospace Exploration Agency), will do this using an instrument called Resolve.

XRISM is scheduled to launch from Japan’s Tanegashima Space Center on Aug. 25, 2023 (Aug. 26 in Japan).

“Resolve will give us a new look into some of the universe’s most energetic objects, including black holes, clusters of galaxies, and the aftermath of stellar explosions,” said Richard Kelley, NASA’s XRISM principal investigator at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “We’ll learn more about how they behave and what they’re made of using the data the mission collects after launch.”

Resolve is an X-ray microcalorimeter spectrometer instrument collaboration between NASA and JAXA. It measures tiny temperature changes created when an X-ray hits its 6-by-6-pixel detector. To measure that minuscule increase and determine the X-ray’s energy, the detector needs to cool down to around minus 460 Fahrenheit (minus 270 Celsius), just a fraction of a degree above absolute zero.

The instrument reaches its operating temperature after a multistage mechanical cooling process inside a refrigerator-sized container of liquid helium.

By collecting thousands or even millions of X-rays from a cosmic source, Resolve can measure high-resolution spectra of the object. Spectra are measurements of light’s intensity over a range of energies. Prisms spread visible light into its different energies, which we know better as the colors of the rainbow. Scientists used prisms in early spectrometers to look for spectral lines, which occur when atoms or molecules absorb or emit energy.

Now astronomers use spectrometers, tuned to all kinds of light, to learn about cosmic objects’ physical states, motions, and compositions. Resolve will do spectroscopy for X-rays with energies ranging from 400 to 12,000 electron volts by measuring the energies of individual X-rays to form a spectrum. (For comparison, visible light energies range from about 2 to 3 electron volts.)

“The spectra XRISM collects will be the most detailed we’ve ever seen for some of the phenomena we’ll observe,” said Brian Williams, NASA’s XRISM project scientist at Goddard. “The mission will provide us with insights into some of the most difficult places to study, like the internal structures of neutron stars and near-light-speed particle jets powered by black holes in active galaxies.”

The mission’s other instrument, developed by JAXA, is called Xtend. It will give XRISM one of the largest fields of view of any X-ray imaging satellite flown to date, observing an area about 60% larger than the average apparent size of the full Moon.

Resolve and Xtend rely on two identical X-ray Mirror Assemblies developed at Goddard.

XRISM is a collaborative mission between JAXA and NASA, with participation by ESA (European Space Agency). NASA’s contribution includes science participation from the Canadian Space Agency.


SwRI team identifies giant swirling waves at the edge of Jupiter’s magnetosphere


Waves produced by Kelvin-Helmholtz instabilities transfer energy in the solar system

Peer-Reviewed Publication

SOUTHWEST RESEARCH INSTITUTE

KHI at Jupiter 

IMAGE: AN SWRI-LED TEAM IDENTIFIED INTERMITTENT EVIDENCE OF KELVIN-HELMHOLTZ INSTABILITIES, GIANT SWIRLING WAVES, AT THE BOUNDARY BETWEEN JUPITER’S MAGNETOSPHERE AND THE SOLAR WIND THAT FILLS INTERPLANETARY SPACE, MODELED HERE BY UNIVERSITY CORPORATION FOR ATMOSPHERIC RESEARCH SCIENTISTS IN A 2017 GRL PAPER. view more 

CREDIT: UCAR/ZHANG, ET.AL.



SAN ANTONIO — July 17, 2023 —A team led by Southwest Research Institute (SwRI) and The University of Texas at San Antonio (UTSA) has found that NASA’s Juno spacecraft orbiting Jupiter frequently encounters giant swirling waves at the boundary between the solar wind and Jupiter’s magnetosphere. The waves are an important process for transferring energy and mass from the solar wind, a stream of charged particles emitted by the Sun, to planetary space environments.

Jake Montgomery, a doctoral student in the joint space physics program between UTSA and SwRI, noted that these phenomena occur when a large difference in velocity forms across the boundary between two regions in space. This can create a swirling wave, or vortex, at the interface that separates a planet’s magnetic field and the solar wind, known as the magnetopause. These Kelvin-Helmholtz waves are not visible to the naked eye but can be detected through instrument observations of plasma and magnetic fields in space. Plasma — a fundamental state of matter made up of charged particles, ions and electrons — is ubiquitous across the universe.

“Kelvin-Helmholtz instabilities are a fundamental physical process that occurs when solar and stellar winds interact with planetary magnetic fields across our solar system and throughout the universe,” Montgomery said. “Juno observed these waves during many of its orbits, providing conclusive evidence that Kelvin-Helmholtz instabilities play an active role in the interaction between the solar wind and Jupiter.”

Montgomery is the lead author of a study published in Geophysical Research Letters that uses data from multiple Juno instruments, including its magnetometer and the SwRI-built Jovian Auroral Distributions Experiment (JADE).

“Juno’s extensive time near Jupiter’s magnetopause has enabled detailed observations of phenomena such as Kelvin-Helmholtz instabilities in this region,” said Dr. Robert Ebert, a staff scientist at SwRI who also serves as an adjoint professor at UTSA. “This solar wind interaction is important as it can transport plasma and energy across the magnetopause, into Jupiter’s magnetosphere, driving activity within that system.”

The paper “Investigating the Occurrence of Kelvin-Helmholtz Instabilities at Jupiter’s Dawn Magnetopause” appears in Geophysical Research Letters and can be accessed at https://doi.org/10.1029/2023GL102921.

For more information, visit https://www.swri.org/planetary-science.



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